1. Field of the Invention
The field of the invention relates to circuit breakers generally, and more particularly to certain new and useful advances in circuit breakers having a thermal overload release trip system, of which the following is a specification, reference being had to the drawings accompanying and forming a part of the same.
2. Description of Related Art
Circuit breakers having one or more poles are well known electrical devices. In general, the function of a circuit breaker is to electrically engage and disengage a selected monitored circuit from an electrical power supply. Circuit breakers are intended to provide protection in electrical circuits and distribution systems against electrical faults, such as prolonged electrical overload conditions and short-circuit fault currents, by providing automatic current interruption to the monitored circuit when the fault conditions occur. The protection function is accomplished by directing a current from the monitored circuit through a primary current path through each pole of the circuit breaker and, in response to a detected fault condition, rapidly tripping, i.e., releasing a mechanical latching of an operating mechanism to separate a pair of electrical contacts into a “tripped” OFF position thereby breaking the circuit.
Such conventional circuit breakers typically include both a magnetic and a thermal overload release trip system to sense a fault or overload condition in the circuit and to trigger the tripping response.
The thermal overload release type tripping system of conventional circuit breakers responds to electrical currents moderately above the circuit breaker's current rating by providing a delayed trip of the circuit breaker. The thermal overload release conventionally includes a thermally responsive conductive bimetal member that deflects in response to heating. A flexible conductor, such as a braided copper wire, cooperates with the bimetal member and the circuit breaker mechanism to allow operative movement of the bimetal member along the circuit breaker current path.
In many conventional circuit breakers, the bimetal is electrically connected in series with the primary current path through at least one circuit breaker pole and arranged to deflect in response to Joule effect heating, (i.e., caused by the electrical current through it). In some cases, the bimetal is not disposed as part of the current path and is instead coupled to a heater, such as an inductive-type heater, which provides the current-generated heat to the bimetal.
In the event of an overload current, the circuit breaker bimetal deflects such that it causes a tripping mechanism that includes a spring-biased latch assembly to trigger the separation of a movable contact attached to a movable arm away from a stationary contact to a “tripped” OFF state. For example, the bimetal is often configured and positioned such that the deflection of the bimetal drives a pivot arm, which in turn releases a latch. At a predetermined displacement of the bimetal, the latch will release to allow a stored energy device, such as a spring, to cause the separation of the contacts.
For a circuit breaker employing a conventional thermal overload release, a sufficient minimum trip force must be provided to overcome the mechanical latching forces within the circuit breaker operating and tripping mechanisms.
For a conventional circuit breaker pole, the bimetal is connected in the primary current path through the circuit breaker pole and configured to deflect in response to Joule effect heating. In the event of a predetermined thermal condition, the bimetal contacts and displaces a trip bar. The bimetal is also electrically connected at the first end with the flexible conductor. The flexible conductor accommodates the operable movement of the bimetal on the on the primary current path.
Other known circuit breakers have used a bimetal that is not connected in the primary current path through the circuit breaker pole, but is instead heated by a separate heater element (not shown) that is not in the primary current path of the circuit breaker pole.
A known shortcoming of a conventional circuit breaker thermal overload release devices using either a conductive bimetal, or an indirectly heated bimetal, temperature sensing member, is that the bimetal members are prone to calibration issues which result in a high rejection loss during circuit breaker assembly. Additionally, a welding or brazing process is often used to attach the bimetal to the heater, or to attach the braided flexible conductor to the conductive bimetal, which can cause overheating and damage to the bimetal member. Additionally, the maximum force output and displacement (work output) of conventional bimetal members are relatively close to the minimum required trip force of the circuit breaker tripping mechanism, thus resulting in an undesirably narrow output force tolerance range for the bimetal member.
Another shortcoming of prior art bimetal controlled circuit breakers having a bimetal element connected in the primary conducting path of the circuit breaker is that the bimetal element may be overloaded by fault currents that are too high and thus consequently damaged and rendered inoperable.
Additionally, a shortcoming of circuit breakers having indirectly heated bimetal elements (i.e., not connected in series with the primary current path of the circuit breaker pole), being heated by a separate heater element is that the heater represents an additional part having relatively complex geometry that must be provided and thus requires additional cost.
Prior art circuit breakers have also employed a shape memory alloy (SMA) wire material, instead of a bimetal, as the thermally responsive element connected in the conducting path of circuit breakers to deflect in response to Joule effect heating. When a thermally responsive element made of shape memory alloy of a first original shape is formed to a second selected shape, and then is heated, for example by the Joule effect, the member exerts a force in the direction which will bring its shape nearer to the first original shape via a phase transformation (the reversion transformation from the martensite phase to the parent phase). This force tending towards alteration of the second selected shape of the member towards a first original shape that it “remembers” can be utilized for driving a driven member in a desired direction.
Conventionally, the SMA wire is formed into a particular shape, such as by winding into a coil, and the coil is then arranged to remember a first original shape in which it has a particular first length in its longitudinal direction. In one arrangement, for example, in a non-actuated condition of the SMA wire, the coil is biased to have a particular second axial length, and then, when the coil is heated by the passage of an electric current through it, the coil tries to return to the original first length, thus exerting an actuation or tripping force in its longitudinal direction.
At least one known problem with using a directly heated (i.e. heated by the Joule effect) SMA type temperature sensing member connected in series with the primary conducting path of the circuit breaker pole is that relatively large currents in the primary conductive path of the circuit breaker pole often result in damage to the SMA member response to high level current spikes, such as for example in the case of a short circuit condition. Conversely, at least one known problem with using a directly heated SMA type temperature sensing member connected electrically in parallel with the primary conducting path of the circuit breaker pole is that, since a relatively high temperature is required to activate the SMA member, it is difficult to use arrange a secondary high-resistance current path in parallel with the primary conducting path that provides sufficient heat to reach the activation temperature of the SMA member, while simultaneously preventing overly high temperatures that would result in damage to the SMA member. Still another problem preventing use of using SMA members heated via the Joule effect, is SMA materials are difficult to properly attach to other conductors via welding, brazing, or soldering without damaging the SMA material.
Likewise, at least one known problem preventing the use of indirectly heated (i.e. by a separate heating element) SMA type temperature sensing members is that, since a relatively high temperature is required to activate the SMA, it is difficult to use a separate heating element to provide sufficient heat to reach the activation temperature of the SMA member, while simultaneously preventing overly high temperatures that would result in damage to the SMA member and the heater.
Moreover, yet another problem preventing the use of an indirectly heated SMA type temperature sensing member is that the SMA member requires an additional element to hold, or otherwise support the SMA member.
For at least the reasons stated above, a need exists for a circuit breaker having an improved thermal overload trip function.